In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999). Performance may be improved if both the transmitter and the receiver are equipped with multiple antennas to provide a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from an antenna array over a MIMO channel to a wireless terminal, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from different antennas of the base station antenna array. Stated in other words, multiple symbol streams may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same downlink time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station transmitting from the same antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
The performance of a wireless communication system can thus be improved using multiple antennas at the base station and/or wireless terminal to provide spatial multiplexing SM in more favorable channel conditions and to provide transmit diversity gain in less favorable channel conditions. Transmit diversity and/or spatial multiplexing may be implemented without knowledge of the wireless channel at the transmitter. In many wireless communication standards such as the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE), High-Speed Downlink Packet Access (HSDPA), and/or Worldwide Interoperability for Microwave Access (WiMAX), however, knowledge of the wireless channel (referred to as channel state information or CSI) may be provided at the MIMO transmitter via feedback from the receiver as discussed, for example, in the 3rd Generation Partnership Project document entitled “UTRA-UTRAN Long Term Evolution (LTE) And 3GPP System Architecture Evolution (SAE)” (http://www.3gpp.org/Highlights/LTE/LTE.htm). Accordingly, the MIMO transmitter can use the channel state information (or CSI) to provide precoding to further improve system performance as discussed, for example, by Scaglione et al. in “Optimal Designs For Space-Time Linear Precoders And Decoders” (IEEE Transactions On Signal Processing, Vol. 50, No. 5, May 2002, pages 1051 to 1064) and by Sampath et al. in “Generalized Linear Precoder And Decoder Design For MIMO Channels Using The Weighted MMSE Criterion” (IEEE Transactions On Communications, Vol. 49, No. 12, December 2001, pages 2198 to 2206). CSI precoding can thus be used by a MIMO transmitter to provide beam forming gain and/or to condition transmissions to existing characteristics of the wireless channel.
Multiple transmit and receive antennas can thus increase a data carrying capacity of a wireless system. For such multiple-input multiple-output (MIMO) systems, Maximum-Likelihood and/or Maximum A posteriori Probability (ML/MAP) detection using exhaustive search may be difficult to implement because MIMO detector complexity increases exponentially with the number of transmit antennas or/and with the number of bits per constellation point.
Detector structures have been proposed to reduce complexity of MIMO detectors. These proposed detectors can be classified as linear and nonlinear detectors. Linear detectors may include zero-forcing (ZF) and minimum mean-square error (MMSE) detectors, and nonlinear detectors may include decision feedback, nulling-cancelling, and/or variations thereof that rely on successive interference cancellation. These detectors may be relatively easy to implement but their bit error rate (BER) and/or Frame error rate (FER) performance may be less than that of a MIMO detector using exhaustive searching.
A MIMO communication system with N_t transmit antennas may support the simultaneous transmission/reception of N_t transport blocks over a same carrier frequency. Prior to transmission, CRC (cyclical redundancy check) bits may be added to each transport block and passed to a channel encoder which adds parity bits to protect the data. The bit stream of each transport block (including CRC and channel encoder parity bits) is then passed through an interleaver where an interleaver size may be adaptively controlled by puncturing to increase the data rate. More particularly, the interleaver size may be adaptively controlled using information from the feedback channel, for example, using channel state information provided by the remote receiver. The interleaved data for each transport block is then passed through a respective modulator (to provide symbol mapping), and the modulator may also be controlled using information from the feedback channel. The resulting symbol stream for each transport block, for example, may then be subjected to an Inverse Fast Fourier Transform (IFFT) before transmission from a respective MIMO antenna. IFFT may be used for transmissions in communication systems which implement OFDMA (orthogonal frequency division multiple access) as the access technology (e.g., LTE, LTE-A, Wi-max, etc.). For other systems which implement CDMA (code division multiple access) as the access technology (e.g., HSDPA, etc), IFFT may be replaced by spreading/scrambling.
For each receiver antenna in an OFDMA access technology system, a Fast Fourier Transform (FFT) operation may be performed for each receiver antenna in a radio frequency front end to generate a stream of discrete digital baseband samples for each receiver antenna. In systems using other access technologies, baseband samples may be generated using other operations. For example, a despreading/descrambling operation may be used to generate baseband samples in a CDMA access technology.
A linear MMSE detector may then be used to generate respective symbol streams for each of the transport blocks with reduced multi antenna interference. A demodulator (also referred to as a de-mapper) may then compute bit log likelihood ratios from the MMSE symbol outputs to provide respective bit streams for each of the transport blocks. The bit stream for each transport block is then de-interleaved and subject to channel decoding, and a CRC check is done on the output of the channel decoder for each transport block. For each transport block, if the transport block passes the cyclic redundancy check (CRC), an ACK (acknowledge message) for the passing transport block is sent to the transmitting device (i.e., the device that transmitted the passing transport block) via a feedback channel. If the cyclic redundancy check for a transport block fails, then a NAK (negative acknowledge message) for the failing transport block is sent to the transmitter (i.e., the device that transmitted the failing transport block). Each transport block may thus be transmitted/retransmitted from the transmitting device until an ACK is received from the receiving device.
In a conventional cancellation algorithm, if one transport block passes CRC and another transport block fails CRC, the passing transport block may be reconstructed, and the reconstructed passing transport block may be used to reduce interference caused by the passing transport block at an input of the MMSE detector relative to the failing transport block. This conventional cancellation may work well for 2×2 MIMO systems (i.e., systems with 2 transmit antennas and 2 receive antennas supporting 2 transport blocks), but may be inefficient for higher order MIMO antenna systems (e.g., 4×4 systems with 4 transmit and 4 receive antennas supporting 4 transport blocks, in 8×8 systems with 8 transmit antennas and 8 receive antennas supporting 8 transport blocks, etc.). Moreover, the use of a decoder (e.g., a turbo decoder) may be relatively complex (e.g., requiring numerous multiplication operations).
In MIMO systems supporting the simultaneous transmission of two or more transport blocks, however, conventional a cancellation algorithm may be relatively inefficient when more than one transport block fails the cyclic redundancy check.